The invention relates to a display device as defined in the pre-characterizing part of claim 1.
A display device of the type mentioned in the opening paragraph is known from U.S. Pat. No. 4,113,360.
Said patent describes a display device comprising a first plate of a fluorescent material, in which, in operation, light is generated and trapped (so that this plate forms a light guide), a second plate which is situated at some distance from the first plate and, between said two plates, a movable element in the form of a membrane. By applying voltages to addressable electrodes on the first and second plates and an electrode on the movable element, the movable element can be locally brought into contact with the first plate, or the contact can be interrupted. A transparent contact liquid is present on the contact surfaces. At locations where the movable element is in contact with the first plate, light is decoupled from said first plate. This enables an image to be represented. If the movable element is not in contact with the light guide, it is in contact with the second plate.
For a proper functioning of the display device, it is important that, on the one hand, the contact between the light guide and the movable element can be brought about and interrupted in an accurate and reliable manner, and that, on the other hand, the design is simple and does not require much energy to operate.
It is an object of the invention to provide a display device of the type mentioned in the opening paragraph, which provides a simple and yet reliable device.
To achieve this object, the display device in accordance with the invention is defined in claim 1.
In the known device, the position of the movable element, i.e. whether or not it makes contact with the light guide, is dependent on the applied voltages and on said voltages only. Positioning of the common electrode on the second plate allows a robust way of moving the element back and forth between the light guide and the second plate. In this way, selection of the movable element becomes independent of a force acting on the movable element directed away from one of the plates. In practice, this force may be influenced by local variation of, for example, the surface of the plates, the surface of the foil or the spacers. The side of the movable element, which side is in optical contact with the light guide, is very flat and smooth. The other side is much rougher. The largest variation will then be in the force which pulls the movable element away from the light guide. If this pulling force is smaller than the Van der Waals force between the surface of the light guide and the movable element and the electrostatic force between the surface of the light guide and the movable element, the movable element will stick to the light guide. Further advantageous embodiments of the invention are defined in the dependent claims.
A special embodiment of the display device in accordance with the invention is defined in claim 2. The forces acting on a movable element are not only dependent on the applied voltages, but also on other forces acting on the element and on its position vis-xc3xa1-vis the electrodes. Said position is also dependent on the history of the element, i.e. previously applied voltages and position. The electric forces acting on the movable element are non-linearly dependent on the distances between the movable element and the electrodes. Because of the non-linear relationship between force and distance, the device exhibits a memory effect. When the movable element is near one of the electrodes, only a relatively large voltage difference between the electrodes can move the element to the other electrode. This, however, also means that once a movable element is in a certain position, it will stay in such a position, even if the voltages applied are changed, provided that they do not change to such a large degree that the movable element is moved to the other electrode. Since the device exhibits a xe2x80x98memory effectxe2x80x99, it is not only the momentary voltages applied which determine whether or not the movable element moves, but this is also determined by previously applied voltages. Using this insight, one or a number of advantages can be obtained. The device can be simplified, and/or the addressing voltages applied to the device can be simplified and/or the energy required can be lowered and/or the reliability of the device can be increased. Also grey levels can be made, as will be explained.
A further embodiment of the device in accordance with the invention is defined in claim 3. This configuration of the row and column electrodes allows a more economic power consumption because the total capacitance formed by the column electrodes and the row electrodes is usually smaller than the situation where the rows are situated on the active plate and the columns are situated on the movable element, because the position of the movable elements in the configuration is mostly directed towards the second plate.
A further embodiment of the device in accordance with the invention is defined in claim 4. Application of the upper value to the lower column electrode alone does not actuate the movable element at the crossing area of the relevant row and column electrodes. Only simultaneous application of the lower value to the column electrodes, and the upper value to the row electrode will actuate the element at the crossing areas. Actuating the movable elements becomes very reliable by this measure. Small deviations of applied voltages do not inadvertently switch an element. Basically, application of an xe2x80x98onxe2x80x99 signal to the row electrode will turn a pixel xe2x80x98onxe2x80x99 when it is xe2x80x98offxe2x80x99.
A further embodiment of the device in accordance with the invention is defined in claim 5. In this way, the electrostatic force between the common electrode on the second plate and the row electrode on the movable element at the selected area becomes equal to zero and hence the reliability of the device is improved because the movement of the movable element away from the light guide is made independent of the variations in the pulling force.
A further embodiment of the device in accordance with the invention is defined in claim 6. Simultaneous application of two xe2x80x98offxe2x80x99 signals to row and column electrode(s) will turn a pixel xe2x80x98offxe2x80x99 when it is xe2x80x98onxe2x80x99, as will be further explained in the description.
A further embodiment of the device in accordance with the invention is defined in claim 7. In this way, the electrostatic force at the selected area between the row electrode on the movable element and the column electrode on the light guide area becomes equal to zero and hence the reliability of the device is improved because the movement of the movable element away from the light guide is made independent of the variations in the pulling force.
A further embodiment of the device in accordance with the invention is defined in claim 8. A turn-on addressing voltage is understood to mean a voltage value which, when combined with a given voltage at a crossing electrode, results in bringing the movable element into contact with the light guide at the crossing area. Likewise, a first turn-off voltage is understood to mean a voltage value which, when combined with a second turn-off voltage at a crossing row electrode, results in releasing the movable element from the light guide at the crossing area. This embodiment is based on the following recognition. When the first row electrode is supplied with an xe2x80x98onxe2x80x99 signal (turn-on voltage) and the crossing row electrodes are supplied with a predetermined voltage, pixels corresponding to areas where electrodes cross will be turned xe2x80x98onxe2x80x99 The step thereafter is used to supply the first turn-off voltage to a second set of column electrodes and to supply a second turn off voltage to the row electrode to bring the movable element at selected areas of the first row crossing the column electrodes back to the second plate after a first short interval. This means that the first line of picture elements remains visible, i.e. xe2x80x98onxe2x80x99. After a second interval, the first turn-off voltages are supplied to all column electrodes and the second turn-off voltage is supplied to the row electrode. This will bring the movable element at all the crossing areas relating to the first row crossing the column, back to the second plate. The second interval relates to the brightness of the selected crossing areas corresponding to the information to be displayed. In this form, a single line of picture elements is displayed. It will be clear that this scheme can be expanded to more than 2 lines.
The great advantage is that, while the second (or third etc.) line of picture elements is formed, the first (second etc.) line of picture elements remains xe2x80x98onxe2x80x99. The total intensity of the light is thereby increased substantially in comparison with arrangements in which (as, for instance, in classical CRTs) only one line of picture elements (or pixels) is activated (xe2x80x98onxe2x80x99) at any one time.
This allows multi-line operation, i.e. more than one line (multi-line) is simultaneously active. The lines of picture elements (the video information) could be written in columns or rows. This also allows grey levels to be made.
A row or column electrode is active between the time when a turn-on voltage has been supplied to the row or column electrode until a turn-off voltage has been supplied to said row or column electrode.